display proper message with proper errno for kvm. (sonic-net#3750)

* display proper message with proper errno for kvm.

* this change should cover all flags.

* add unit test.

* fix typo.

* fix unit test.

* whitespace EOF

* delete dead variable.

Author: hdwhdw

scripts/decode-syseeprom

@@ -14,6 +14,7 @@ import optparse
 import os
 import re
 import sys
+import errno
 
 import sonic_platform
 from sonic_platform_base.sonic_eeprom.eeprom_tlvinfo import TlvInfoDecoder
@@ -238,6 +239,12 @@ def main():
     if any(re.match(p, platform) for p in platforms_without_eeprom_db):
         support_eeprom_db = False
 
+    # Currently, kvm does not support eeprom
+    platforms_without_eeprom = ['.*kvm.*']
+    if any(re.match(p, platform) for p in platforms_without_eeprom):
+        print('Platform {} does not support EEPROM'.format(platform))
+        return errno.ENODEV
+
     use_db = opts.db and support_eeprom_db
 
     if opts.mgmtmac:

tests/decode_syseeprom_test.py

@@ -1,5 +1,6 @@
 import os
 import sys
+import errno
 from unittest import mock
 
 import pytest
@@ -202,3 +203,12 @@ def test_support_platforms_not_db_based(self, mockDbBased, mockNotDbBased):
         decode_syseeprom.main()
         assert mockNotDbBased.called
         assert not mockDbBased.called
+
+    @mock.patch('os.geteuid', lambda: 0)
+    @mock.patch('sonic_py_common.device_info.get_platform', lambda: 'kvm')
+    @mock.patch.object(sys, 'argv', ["decode-syseeprom"])
+    @mock.patch('decode_syseeprom.read_and_print_eeprom')
+    @mock.patch('decode_syseeprom.read_eeprom_from_db')
+    def test_support_platforms_no_eeprom(self, mockDbBased, mockNotDbBased):
+        ret = decode_syseeprom.main()
+        assert ret == errno.ENODEV